This invention relates generally to light sources. More particularly, it relates to light sources for image display systems.
Systems for displaying color images are based on combining typically three or more primary colors of light, such as red, blue and green. In the prior art, cathode ray tubes (CRT""s) with phosphor screens provided most color images for television and computer monitors. The image is built up from a series of pixels on a phosphor screen. Each pixel on the screen contains phosphors that produce different colors of light when struck by an electron beam. Typically, red, green and blue phosphors are placed close to each other at the location of each pixel. Scanning the beam across the screen while modulating beam intensity produces color images. Although cathode ray tube technology is widely used for television and computer monitors, the electron gun that produces the electron beam requires a lot of power. The electron gun and beam must be enclosed in a vacuum tube. The deflection of the electron beam requires considerable space, which adds to the space occupied by the display. Furthermore, CRT systems for projecting images on a screen are awkward and produce inferior image quality.
Recently light emitting diodes (LEDs) have become popular for illuminating flat panel displays in laptop computers and video cameras. These displays consume less power and take up less space than cathode ray tube displays. Unfortunately, the brightness output of typical LED""s is limited. Large area LED displays require large numbers of LED""s, which adds to the complexity and cost of the display.
Laser based displays offer an alternative to CRTs and LEDs. A laser image display system typically comprises laser sources, modulators, combining optics and a scanner. The laser source produces one or more beams of laser light. Separate sources provide laser light having free-space wavelengths corresponding to the colors red, green and blue. The modulators vary the intensity of each beam of light to vary the color of each pixel in the image. The combining optics joins the red, green and blue beams to form a single beam. The scanner deflects the beam, which is projected onto a screen to form an image.
Such laser based image displays are more compact than CRT displays and provide better image quality. However, there are still problems. One problem has been the production of blue laser light. There are few practical laser process that produce blue laser light.
Recently, diode lasers and non-linear optical devices have become available for the production of blue light. Non-linear devices include second harmonic generators, optical parametric generators and sum frequency generators.
Second harmonic generators take two identical photons of input light and produce a single output photon having the energy of the two input photons. The output photon has twice the frequency of the input photons and, therefore, half the wavelength. A second harmonic generator may be used, for example, to take infrared light at a free-space wavelength of 1064-nm and produce green light at 532-nm. Optical parametric generators (OPGs) split an input photon into two photons having different free-space wavelengths. For example, an OPG can split a beam of green light having a free-space wavelength of 526.5-nm into two beams of infrared light having free-space wavelengths of 1240-nm and 915-nm. Generally, the shorter wavelength (i.e., higher frequency and therefore higher energy) beam is referred to as a signal and the longer wavelength beam is referred to as an idler.
Sum frequency generators take two photons having different frequencies and produce an output photon having a frequency equal to the sum of the frequencies of the two input photons. For example, a sum frequency generator could take green light having a free-space wavelength of 532-nm and combine it with infrared light having a free-space wavelength of 3.42 microns to produce blue light having a free-space wavelength of 460-nm.
Three-color laser display systems based on non-linear optical devices are described in U.S. Pat. Nos. 5,828,424 and 5,740,190. However, each of these displays requires combining optics to combine the red, blue and green beams into a single beam for projecting the image. If the combining optics do not properly align the three beams so that they are collinear, the image quality degrades.
There is a need, therefore, for an improved laser based display that overcomes the above difficulties.
Accordingly, it is a primary object of the present invention to provide an improved laser based source of red, green and blue light. It is a further object of the invention to provide a source that generates coherent red, green, and blue light beams collinearly. It is an additional object of the invention to provide an improved laser based display system. It is a further object of the present invention provide a light source that is more efficient at producing blue light.
These objects and advantages are attained by an apparatus for generating three or more optical signals. According to a first embodiment, The apparatus generally comprises a coherent source, an optical parametric generation device and a sum frequency generator. The coherent source generates a first optical signal characterized by a first free-space wavelength. The optical parametric generation device interacts with a first portion of the first optical signal to produce a second optical signal characterized by a second free-space wavelength. The optical parametric generation device also produces an idler signal and transmits a second portion of the first optical signal to the sum frequency generator. The sum frequency generator non-linearly combines part of the second portion of the first optical signal with the idler signal to produce a third optical signal characterized by a third free-space wavelength. The sum frequency generator transmits the second optical signal, the third optical signal and a remainder of the first optical signal. The second harmonic generator may be incorporated with the optical parametric device and the sum frequency generator into a single monolithic structure. The structure may include a resonant cavity to enhance the output of any or all of the three signals.
According to a second embodiment, the first, second, and third optical signals may comprise collinear red green and blue beams that may be modulated and scanned to produce an image. Furthermore, the optical parametric generation device and the sum frequency generator may be fabricated as a single monolithic device.
In an third embodiment, pump radiation from a source interacts with an optical parametric oscillator (OPO) to produce signal radiation having a desired free-space wavelength. Two second-harmonic generators then double the frequency of the signal radiation twice to produce blue light.
In a fourth embodiment pump radiation from a source interacts with an optical parametric oscillator (OPO) to produce signal radiation having a desired free-space wavelength. The pump radiation then combines with the signal radiation in a sum-frequency generator to produce blue light.